A synthetic aperture radar (SAR) is a moving platform radar mapping technique for generating high resolution images of terrain or targets by taking advantage of the platform's relative velocity with respect to the ground below. SAR imaging techniques are used in both aircraft based and space-based systems. In SAR imaging, the ground is illuminated by transmit pulses that are plane polarized, and the echo pulses scattered back towards the receiver carries information about the nature of the features illuminated. The information available to the receiving system is carried in two orthogonal polarization states (e.g., horizontal and vertical).
There is current interest in space-based foliage penetration SARs to extend the utility of this remote sensing technique. Because of their longer wavelengths, low frequency SARs (VHF/UHF band) provide enhanced foliage and ground penetration capabilities as compared to high frequency SARs (L-band and above). However, to date the development and performance of low frequency SARs has been severely limited by the distortion of the RADAR waveform by the ionosphere. This is why low frequency SAR systems generally are operated from aircraft flying below the ionosphere, whereas space-based SARs typically use higher frequencies because ionospheric distortion is generally negligible at shorter wavelengths.
The ionosphere is defined to be the region of the upper atmosphere that includes large quantities of charged particles, such as electrons. The electron density is measured by a quantity termed the total electron content (TEC). In the presence of a magnetic field such as the earth's, the TEC of the ionosphere exhibits a spatially varying value. As a function of the TEC, an information bearing signal (i.e., a RADAR waveform) is both dispersed and rotated as it propagates on a trans-ionospheric path. The dispersion and rotation vary across the spatial expanse that defines a typical synthetic aperture. Variations in the TEC experienced by a low frequency SAR operating in or through the ionosphere result from many variables, including location (i.e., latitude/longitude), diurnal/seasonal variations and solar activity. Even mild ionospheric conditions (e.g., 2 TEC units with a 5% variation) may result in blurring of up to hundreds of meters in the SAR image, in both range and cross-range.
Two important effects of the ionosphere on polarized transmitted pulses and associated echo pulses in the VHF/UHF band can be described in terms of non-constant group delay and Faraday rotation. Non-constant group delay manifests itself as a time-domain pulse spreading and shape distortion due to the non-linear phase characteristic imposed by the ionosphere.
For instance, two-way group delays of the ionosphere in the 200 to 400 MHz frequency band are illustrated in FIG. 1a. Lines 20, 22, 24, 26 and 28 respectively correspond to TEC units of 1, 10, 20, 40 and 60. As the electron density increases, the group delay for a given frequency increases. The corresponding pulse spreading is illustrated in FIG. 1b, wherein lines 30, 32, 34, 36 and 38 respectively correspond to the same TEC units of 1, 10, 20, 40 and 60. Likewise, as the electron density increases, the pulse spreading increases.
Faraday rotation in the ionosphere causes the polarization of transmitted pulses to undergo a rotation. Because of the unknown TEC on the ionospheric path, this rotation is initially unknown, and therefore it is difficult to predict what polarization the pulses will have when they strike an object being imaged. Incident polarization control is a necessary feature for a special class of SARs known as fully polarimetric. The Faraday rotation induced by the ionosphere, if left uncompensated, renders this mode unavailable to low frequency space-based SARs. Further, the illuminated object can induce a polarization rotation of its own. Then on the return path the ionosphere again places an initially unknown rotation on the echo pulses. The polarization shift induced by the illuminated object is actually information that should be preserved for a fully polarimetric SAR. Even for traditional (non-polarimetric) SAR applications, it is of great interest for polarization states induced by the propagation medium to be known at the radar transmitter/receiver so the data can be properly processed and interpreted by end users, such as image analysts.
Ionospheric rotation angles versus frequency in the 200 to 400 MHz frequency band are illustrated in FIG. 2. Lines 40, 42, 44, 46, 48, 50 and 52 respectively correspond to TEC units of 1, 4, 10, 20, 40, 60 and 100. As the electron density increases, the Faraday rotation angle increases. At a frequency of 200 MHz, the Faraday rotation causes phase ambiguities (i.e., more than 360-degree rotations) for electron densities greater than 20 TEC units. Even when the rotation is too small to generate a phase ambiguity, it may significantly impact both the incident and reflected field at each scatterer in the scene.
The ionospheric effects of non-linear group delay and Faraday rotation may be compensated using a number of known techniques including predictive techniques, data adaptive techniques and scene aided techniques. Predictive techniques use measurements of space weather, GPS frequency shifts, and UV imaging. For example, signals from Global Positioning System (GPS) satellites can be used to measure current ionospheric distortions in particular locations. These measurements can be used to predict ionospheric conditions throughout the world, and these predictions can then be broadcast world-wide. The quality of the prediction is generally insufficient for purposes of a spaced-based SAR, because it is very difficult to obtain measurements with sufficient temporal and spatial proximity to the SAR platform. Other techniques for direct ionospheric measurement exist (i.e., ionosondes), but all are plagued by the same limitations as the GPS method.
Common data adaptive techniques include phase gradient autofocus, inverse filtering and map drift. Here, the ionospheric distortion is measured or estimated based upon the collected data itself. The correction adapts to the specific conditions prevailing on the collection, and primarily deals with systematic errors that are constant across the entire collection. Systematic errors include a fixed timing offset or a mispointed antenna, for example. Since the ionosphere is not homogeneous, the distortion varies with location since the SAR platform is moving. These known data adaptive techniques do not adequately compensate for the spatially varying ionospheric distortion on the transmit pulses and associated echo pulses.
Scene-aided techniques involve receiving signals from artificial (i.e., man-made), ground-based calibration devices with known scattering or transmission properties. Accuracy is limited by the precision with which the ground-based calibrators can be built, maintained and oriented. Scene-aided techniques are constrained from a practical standpoint by the need to locate the calibrators in areas where the calibrators can be clearly detected, which may be difficult in certain areas of interest.
A calibration technique for removing Faraday rotation effects in SAR imagery is disclosed in an article by William Gail titled “Effect Of Faraday Rotation On Polarimetric SAR,” IEEE Transactions on Aerospace and Electronic Systems, Vol. 34, No., 1, January 1998. Gail assumes a single rotation angle to calibrate an entire image, and then evaluates the impact of deviations from this value on the range and azimuth impulse responses. Since the ionosphere is not homogeneous and the distortion is spatially varying, assuming a single rotation angle for an entire image is generally not sufficient for high resolution imaging. Moreover, the article does not address the non-linear phase response of the ionosphere causing time domain distortions.